Star
A star is a massive, self-luminous sphere of plasma held together by its own gravity. It shines by converting hydrogen into helium through nuclear fusion in its core, releasing energy across the electromagnetic spectrum.
Stars form in large clouds of gas and dust—called nebulae—where regions collapse under gravity, creating protostars. When core temperatures reach millions of degrees, fusion ignites, marking the birth of a star. Observatories like Hubble and missions such as NASA’s Infrared telescopes have imaged this process in action.
The majority (~90%) of stars are main-sequence stars, fusing hydrogen into helium. These include a broad range of masses—from red dwarfs (small, long-lived, faint) to blue giants and supergiants (massive, hot, and short-lived). Our Sun is a middle-aged G-type main sequence star.
As stars exhaust their hydrogen fuel, their evolution depends on mass. Lower-mass stars become red giants then white dwarfs. More massive stars undergo successive fusion stages, end in supernova explosions, and leave behind neutron stars or black holes.
Stars vary in brightness, size, and color. They are classified using spectral types (O, B, A, F, G, K, M) based on surface temperature and absorption lines. For example, O- and B-type stars are hot and blue; M-type are cool and red.
Stellar remnants include white dwarfs (Earth-sized cores of former stars), neutron stars (city-sized remnants of supernovae), and black holes (extreme-density objects from the most massive stars).
Stars are not static—many rotate, exhibit magnetic activity (like sunspots and flares), and broadcast stellar winds. Their lifecycle enriches the interstellar medium with heavier elements, seeding future generations of stars and planets.
Stars often exist in groups—binary or systems within star clusters and galaxies. Their properties are studied via brightness, spectra, parallax, variability, and statistical surveys by missions like Gaia and Kepler.
Sources
APODs including "Star"
Cygnus Without Stars
24 April 2008
The sky is full of hydrogen, though it can take a sensitive camera and telescope to see it. For example, this twelve-degree-wide view of the northern part of the constellation Cygnus reveals cosmic clouds of hydrogen gas along the plane of our Milky Way galaxy. The mosaic of telescopic images was recorded through an h-alpha filter that transmits only visible red light from glowing hydrogen atoms. Further digital processing has removed most of what is left of the myriad, point-like Milky Way stars from the scene, though bright Deneb, alpha star of Cygnus and head of the Northern Cross, remains near top center. Recognizable bright nebulae include NGC 7000 (North America Nebula), and IC 5070 (Pelican Nebula) at the upper left with IC 1318 (Butterfly Nebula) and NGC 6888 (Crescent Nebula) at lower right -- but others can be found throughout the wide field. Want the stars back? Just slide your cursor over the picture.